Dear Colleagues:
I am happy to announce that the GGUM2000 software
system is now available free of charge. The GGUM2000
system estimates parameters for a family of item response
theory models for unfolding. The most general model
implemented in the system is the generalized graded
unfolding model (GGUM) that was described in the March
2000 issue of Applied Psychological Measurement (pp. 3-
32). In addition to this very general model, the
GGUM2000 system also estimates seven other models that
can be obtained by constraining item parameters from the
GGUM in alternative ways. The system estimates item
parameters using marginal maximum likelihood, and
person parameters are estimated using an expected a
posteriori (EAP) technique. The program allows for binary
or polytomous responses, up to 100 items with 2-10
response categories, and up to 2000 respondents. The
GGUM2000 system is a DOS-based program and is
accompanied by an informative user's manual in
WordPerfect 6.1 for Windows format. The program can be
downloaded from a web site devoted to IRT models for
unfolding. The site is located at:
http://www.education.umd.edu/EDMS/tutorials/index.html
To obtain the software, click on the "Free Software to
Construct IRT Unfolding Models" and you will be taken to
the GGUM2000 advertisement. Click on "Download
GUMIT2.EXE", then do the same thing on the next screen
that appears.
The GGUM2000 system is supported by the author.
Your feedback is appreciated and will be used to improve
subsequent versions of the system.
While you are at the web site, please notice the other
features available to you. There is an extensive reference
page that provides a current list of books and articles on
IRT-based approaches to unfolding. There is also an
example data sets page from which illustrative test data
may be downloaded. Finally, there is a listing of
commercially available IRT-based unfolding software.
I hope you will stop by the web site soon and get your
free copy of GGUM2000. For those readers who may not
be familiar with IRT models for unfolding, I have included
a clip from the user's manual below. Although it has
increased the length of this post substantially, I hope some
folks find it useful.
Best Wishes,
Jim Roberts
What is GGUM2000?
The GGUM2000 system is a software package that
estimates parameters from a family of item response theory
(IRT) models known as "unfolding models". These models
assume that persons and items can be jointly represented as
locations on a latent unidimensional continuum. A single-
peaked, nonmonotonic response function is the key feature
that distinguishes unfolding IRT models from traditional,
"cumulative" IRT models. This response function suggests
that a higher item score is more likely to the extent that an
individual is located close to a given item on the underlying
continuum. In contrast, cumulative IRT models imply that
a higher item score is more likely when the location of the
individual exceeds that for the item on the latent
continuum.
The unfolding IRT models implemented in the
GGUM2000 system are appropriate for measuring a variety
of constructs. For example, the models are well suited to
measure individual attitudes using data from either
Thurstone or Likert attitude questionnaires (Andrich, 1996;
Roberts, 1995; Roberts, Laughlin & Wedell, 1999). With
these questionnaires, respondents indicate how much they
disagree or agree with each statement. The response may
be binary (0=disagree, 1=agree) or graded (0=strongly
disagree, 1=disagree, 2=slightly disagree, 3=slightly agree,
4=agree, 5=strongly agree), but in each case, higher levels
of agreement are coded with successive integers. In the
context of attitude measurement, these unfolding models
predict more agreement to the extent that an individual's
opinion is similar to the sentiment expressed by the item.
The individual's location on the continuum is a measure of
the individual's attitude and the item's location is a
measure of its sentiment (i.e., its scale value).
These unfolding models are also relevant to preference
measurement situations where a respondent indicates how
much he/she prefers each stimulus in a set of I stimuli.
Suppose preference judgments are obtained from a sample
of respondents using a rating scale with 0 to C scale points
where a response of C represents the highest degree of
preference. In this situation, one might postulate that
respondents and stimuli are jointly located on a
unidimensional continuum. The location of a given
respondent represents the respondent's "ideal point". A
respondent is expected to prefer a stimulus to the extent
that it is located close to this ideal point.
Finally, the unfolding models implemented in the
GGUM2000 system can be used to measure developmental
processes that occur in stages (Noel, 1999). For example,
individuals who attempt to quit smoking may proceed
through a series of cognitive/behavioral stages ranging
from precontemplation about the dangers of smoking, to
contemplation about smoking hazards and the need to quit,
to actively resisting the urge to smoke. Statements
representing each of these stages could be presented on a
questionnaire, and respondents could be asked to indicate
their level of agreement with each statement. An unfolding
model could be used to order the statements along a
unidimensional developmental continuum that illustrates
these stages of change. Additionally, the model could also
estimate the locations of respondents on this continuum.
***************************************************
James S. Roberts, Ph.D.
Assistant Professor
Department of Measurement, Statistics & Evaluation
University of Maryland
College Park, MD 20742
(301) 405-3630 Voice
(301) 314-9245 Fax
**************************************************
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